1. Field of the Invention
The present invention relates to a phosphor thin film, a manufacturing method thereof, and an electroluminescent (EL) panel.
2. Related Background Art
Thin-film EL panels have been actively studied in recent years as light emitting elements for small-size, or large-size and light-weight flat panel display units. An EL panel of a double insulation structure using a thin-film insulation layer is known as an EL panel having a phosphor thin film made of manganese-added zinc sulfide which emits yellow-orange light. A monochrome thin-film EL display unit using the above-described EL panel has been in practical use.
To be more precise, in this EL panel, lower electrodes of a given pattern are formed on a substrate and a first insulating layer is formed on the substrate where these lower electrodes are formed on. Moreover, a phosphor thin film and a second insulating layer are serially formed on this first insulating layer, and upper electrodes are formed in a given pattern on the second insulating layer so as to constitute a matrix circuit together with the lower electrodes. To increase luminance of the phosphor thin film, it is common to anneal the phosphor thin film at a temperature not higher than a strain point of the glass substrate.
Meanwhile, an EL panel with a structure using a ceramic substrate as the substrate and a thick-film dielectric layer as the insulating layer is proposed recently. Moreover, there is also proposed a structure using a high dielectric constant BaTiO3 thin plate as the substrate while forming electrodes on the rear side of the substrate, and thereby using the thin plate as the insulating layer and as the substrate at the same time. Since the ceramics such as alumina or BaTiO3 are used for the substrates in these structures, it is possible to perform high-temperature annealing of the phosphor thin film to increase the luminance. Moreover, since the thick film or the thin-plate dielectric layer is used as the insulating layer, these structures are more resistant to dielectric breakdown and therefore more reliable than the EL panel using the thin film as the insulating layer. Here, it is not always necessary to apply a structure of sandwiching the phosphor thin film between the insulating layers as in the double insulation structure. For the insulating film, it is sufficient if either the thick film or the thin-plate dielectric layer is provided on one side.
In order to apply the display unit using the above-described EL panel to a color display for a personal computer, a television or other display devices, it is essential to colorize the EL panel.
Sulfide phosphor thin film or the like is known as typical phosphor thin films for an EL panel which emit the three primary colors of red, green, or blue light. The thin-film EL panel using the sulfide phosphor thin films has excellent reliability and environmental resistance. As examples of sulfide phosphor thin film, SrS:Ce using SrS as a matrix material (base material) and Ce as a luminescence center, and ZnS:Tm are known as blue light-emitting phosphors. Meanwhile, ZnS:Sm and CaS:Eu are known as red light-emitting phosphors, and ZnS:Tb, CaS:Ce, and the like are known as green light-emitting phosphors. These phosphors are currently under studies.